1. Field of the Invention
The present invention relates to a method for the production of single crystalline TiO2 flakes, to single crystalline TiO2 flakes produced by this method as well as to the use thereof, in particular as pigments.
2. Description of the Related Art
Titanium dioxide (TiO2) is widely used in various fields such as pigments, photo catalysts, solar cells, UV absorbents and water repellants, and various methods have been developed in order to find appropriate methods for producing TiO2, especially in its rutile crystal form.
For example, Japanese Unexamined Patent Application Publication No. 58-88121 (Patent Document 1) discloses a method to obtain titanium dioxide particles of the rutile type which are grown in a specific crystal axis direction, wherein potassium titanate (K2O.nTiO2) fibers obtained by a flux method are treated with an acid resulting in needle-shaped water containing TiO2-particles which are, in the following, heat treated in order to give rutile TiO2-needles of a very small particle size.
Japanese Unexamined Patent Application Publication No. 04-144918 (Patent Document 2) discloses a method to obtain flaky titanium dioxide particles by dissolving titanium alkoxide and an organic alkaline substance in an organic solvent and forming a film on a belt by coating this solution on a smooth surface, followed by hydrolysis, drying, peeling, and calcination steps.
Japanese Unexamined Patent Application Publication No. 07-157312 (Patent Document 3) discloses a method for obtaining flaky titanium dioxide by acid treatment of potassium di-titanate fibers obtained by a melting method in the presence of a fluxing agent, whereby by means of the acid treatment all of the K+ ions within the crystals are eluted, followed by heat treating the resulting particles to obtain a flake type powder.
However, in the techniques described in these Patent Documents, the obtained particle shape of the titanium dioxide particles may partly not be suitable as pigment, in particular if larger flaky shaped crystals are needed, or the production method should be further improved, especially with respect to energy efficiency, production time and cost.
For example, the titanium dioxide particles obtained by the method described in the Patent Document 1 exhibit a whisker type shape having an extremely fine size (particle diameter of at most 6 μm). Therefore, the use of these particles as pigments is limited, especially with regard to their potential use as highly reflective interference pigments.
Further, although the titanium dioxide particles obtained by the method described in the Patent Document 2 have a flake type shape, they exhibit a polycrystalline structure and the production method is based on organic compounds and solvents which cause high cost and technological efforts and, thus, more complications.
Further, the titanium dioxide particles obtained by the method described in Patent Document 3 exhibit a flake type shape of an appropriate size, but the requirement of two calcination steps takes time and causes high cost and energetic waste in the production procedure.
Thus, the aim of the present invention is to reduce energy cost of the production step and to offer a method to effectively produce single crystalline TiO2 flakes, preferably from inorganic compounds, which are suitable for use as pigments having luster, high refractive index as well as large particle size. A further aim of the present invention is to provide single crystalline TiO2 flakes produced according to said process, as well as to suggest the use thereof.